1. Field of the Invention
The present invention relates to a process for the preparation of an information recording medium for writing (i.e., recording) and/or reading out (i.e., reproducing) information by means of a laser beam having high energy density.
2. Description of Prior Art
Information recording media for recording and/or reproducing information by the use of a laser beam of high energy density have been developed in recent years and are put into practical use. Such recording media have been widely utilized in various fields, for example, as an optical disc such as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file, a micro-image recording medium, an ultramicro-image recording medium, a micro-facsimile, and an optical card.
The conventional optical information recording medium basically comprises a transparent substrate of a plastic or glass material and a recording layer provided on the substrate. As materials of the recording layer, metals such as Bi, Sn, In and Te or semimetals, and dyes such as a cyanine dye, a metal complex compound and a quinone dye are generally known.
Writing (i.e., recording) of information on the recording medium can be conducted, for example, by irradiating the medium with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the recording medium absorbs energy of the beam and rise of temperature locally occurs and, as a result, a chemical or physical change is caused to alter (or change) optical characteristics of the recording layer in the irradiated area, whereby the recording of information is made. Reading of the information from the recording medium is also conducted by irradiating the medium with a laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the change in the optical characteristics of the recording layer.
Recently, an optical disc having an air-sandwich structure for protecting the recording layer has been proposed. The optical disc of air-sandwich structure comprises two disc-shaped substrates, a recording layer provided on at least one of the substrates and two ring-shaped spacers (inner spacer and outer spacer), said two substrates interposing the recording layer being combined with each other in such a manner that a closed space is formed by the two substrates and the two spacers. In such recording medium, the recording layer is kept from direct exposure to an outer air, and recording or reproduction of information is carried out by applying a light of the laser beam to the recording layer through the substrate, whereby the recording layer is generally kept from physical or chemical damage. Further, the surface of the recording layer can be kept from deposition of dust which likely causes troubles in the recording and reproducing procedures.
An information recording medium using a dye as a recording material is excellent in various characteristics required for recording media such as high sensitivity, and additionally the medium has other advantageous features such that the recording layer can be easily formed on the substrate by a conventional coating method. In general, most of dyes including a cyanine dye are low in solubility, so that in the preparation of a coating solution using those dyes, solvents having high dissolving power (solvency) for those dyes such as halogenated hydrocarbons (e.g., dichloromethane and dichloroethane) are generally utilized.
However, a substrate made of a plastic material has poor resistance to those halogenated hydrocarbons, and the surface of the substrate is easily dissolved in the coating solution when the solution is coated over the substrate. As a result, there occur various troubles such that protruded and depressed portions for a tracking groove on the surface of the substrate are eliminated, or a portion of material of the substrate is migrated into the recording layer so as to lower the reflectance of the recording layer. For coping with such troubles, Japanese Patent Provisional Publication No. 59(1984)-217241 states that the plastic substrate is beforehand subjected to an insolubilizing treatment to reduce dissolution by the halogenated hydrocarbon solvent.